1. Field of the Invention
The present invention relates to surface treatment of a metal, and more particularly, to surface treatment of a metal using a phosphate chemical film.
2. Description of the Related Art
To begin with, if phosphate chemical treatment technology were to be divided into electrolytic treatment and non-electrolytic treatment, electrolytic treatment would be a new technology while non-electrolytic treatment would be a conventional technology. Although the reaction of phosphate chemical treatment is an electrochemical reaction for both non-electrolytic treatment and electrolytic treatment, the contents of that reaction are quite different.
The inventor of the present invention previously filed a patent relating to electrolytic phosphate chemical treatment (Japanese Unexamined Patent Publication No. 2000-234200). At the time of the previous filing, a study was conducted relating to electrolytic phosphate chemical treatment of the prior art. However, studies regarding the inherent prior art of non-electrolytic phosphate chemical treatment were not always adequate. To begin with, the difference between the electrochemical reactions of non-electrolytic treatment and electrolytic treatment are clarified with respect to surface treatment. To accomplish this object, the mechanism of the chemical reaction in non-electrolytic treatment is shown in FIG. 8. In contrast, the mechanism of the electrochemical reaction in electrolytic treatment is shown in FIG. 1.
The major differences between non-electrolytic treatment and electrolytic treatment with respect to surface treatment can be summarized as indicated below.
(i) In the case of non-electrolytic treatment, a film is formed by an electrochemical reaction in the same treatment bath and on the same metal surface. Namely, the anode and cathode in the electrochemical reaction are the same metal surfaces. On the other hand, electrolytic treatment involves the application of voltage and current from an external power supply in the same treatment bath. A film is then formed by an electrochemical reaction under conditions in which the electrodes are divided into an anode and cathode. Consequently, the electrochemical reaction in electrolytic treatment is divided into a reaction on an anode and a reaction on a cathode that are separated in a treatment bath.
(ii) In electrolytic treatment, as shown in FIG. 1, a solution is divided into a solution phase and an interface (metal surface). It is necessary that the applied voltage and current be limited to acting only on the interface. As a result, the film forming reaction of the solution component due to electrolysis only acts on the metal surface. In this manner, the phase transition (film formation) from the liquid to solid, which constitutes the deposition of the film, can be limited to only the metal surface. In other words, in electrolytic treatment, it is important to create a mechanism that is capable of preventing a reaction in the solution phase.
On the other hand, in non-electrolytic treatment, although film formation occurs on the surface of an article to be treated, the reaction components are supplied to a location away from the metal surface (solution phase). Namely, in non-electrolytic treatment, a film is formed on the metal surface by allowing the component of the solution phase to react. This is because film formation (phase transition from a liquid to a solid) is carried out more easily on the surface of the article to be treated (metal) than in the solution phase. Consequently, it is not necessary in non-electrolytic treatment to strictly separate the solution phase and interface as compared with electrolytic treatment. From the standpoint of forming a film by controlling an electrochemical reaction, there is a considerable difference between forming sludge by reacting the component of a solution phase and not forming sludge by not allowing to react.
(iii) Difference in Reaction Voltage
The present invention is targeted at film formation from an aqueous solution using water as the solvent. The electrochemical reaction in non-electrolytic treatment does not assume the decomposition of a solvent in the form of water. Consequently, the electrochemical reaction is at a voltage of 1.23 V or less, the decomposition voltage of water. On the other hand, in the case of electrolytic treatment, which uses an external power supply, it is typically accompanied by a decomposition reaction of water (solvent). Consequently, the electrolytic reaction voltage typically exceeds 1.23 V. This difference in the reaction voltage, along with the presence or absence of the accompanying decomposition of solvent (water), are the major differences between electrolytic treatment and non-electrolytic treatment.
Next, an explanation is provided of the prior art with respect to electrolytic treatment.
As an example of the prior art, Japanese Unexamined Patent Publication No. 2000-234200 discloses an electrolytic phosphate treatment method comprising:                forming a film containing a phosphate compound and a metal that is not a phosphate on the surface of an article to be treated having electrical conductivity by performing electrolytic treatment by contacting the article to be treated with a phosphate chemical treatment bath containing phosphate ions and phosphoric acid, nitrate ions, metal ions that form a complex with the phosphate ions in the phosphate chemical treatment bath (e.g., zinc, iron, manganese or calcium ions), and metal ions for which the electrical potential at which the ions dissolved in the phosphate chemical treatment bath are reduced and precipitate as metal is equal to or greater than the cathodic electrolysis reaction potential of the solvent in the form of water or equal to or greater than −830 mV (e.g., nickel, copper or iron ion) based on a reference electrode potential; wherein        the above phosphate chemical treatment bath contains 0-400 ppm of metal ions other those which are a component that forms the above film (e.g., sodium ion), and is substantially free of solids (sludge) having an effect on the film formation reaction; and        the above article to be treated is treated by electrolysis in the above phosphate chemical treatment bath with a metal material that forms a complex with phosphate ions in this treatment bath, and a metal material for which the electrical potential at which ions thereof dissolved in the phosphate chemical treatment bath are reduced and precipitate as metal is, based on a reference electrode potential, equal to or greater than the cathodic electrolysis reaction potential of the solvent in the form of water or −830 mV or higher (indicated as the potential relative to a standard hydrogen electrode), and/or an insoluble electrode material.        
This electrolytic phosphate treatment method of the prior art was developed in order to efficiently form a phosphate-metal mixed chemical film without causing the formation of sludge in the treatment bath. However, when this method is used to carry out treatment continuously, it was found that sludge forms depending on the treatment conditions.
One of the reasons for being unable to practically apply the electrolytic phosphate chemical treatment in Japanese Unexamined Patent Publication No. 2000-234200 is that in phosphate chemical treatment, all three constituent features relating to electrolytic treatment consisting of the solution, counter electrode and article to be treated are involved in the reaction. The following Table 1 is shown in reference to this point.
TABLE 1Classification of Wet Electrolytic Treatment(O: Reacts, X: Does not react)CounterArticle to beelectrodeSolutiontreatedElectroplatingOXXElectrodepositionXOXcoatingElectrolytic phosphateO or XOOchemical treatment
In the electrolytic phosphate chemical treatment of the above-mentioned Japanese Unexamined Patent Publication No. 2000-234200, attention was not paid to “not allowing the components in solution to react at a location other than the electrode surface” in particular. Consequently, corrective actions and accommodations were performed consisting of:                (1) prevention of contamination by impurities (Na ions, etc.)        (2) prevention of self-decomposition and aggregation of solution components by constantly filtering and circulating the treatment, maintaining the temperature and so forth, and        (3) use of a complex.        
However, in the case of performing treatment continuously, it was found to be difficult to maintain “not allowing the components in solution to react at a location other than the electrode surface” with only the accommodations made in the above-mentioned invention of Japanese Unexamined Patent Publication No. 2000-234200. Namely, in Japanese Unexamined Patent Publication No. 2000-234200, although the treatment bath is constantly filtered and circulated during electrolytic treatment, it was found that solids (sludge) are trapped by the filter at that time. The amount captured can be maintained within a range that can be allowed with respect to film formation in terms of practical application of this method. However, this sludge becomes partially redissolved (for example, Zn2Fe(PO4)2+6H+→2H3PO4+2Zn2++Fe2+). This phenomenon (reaction) impairs film formation. Thus, it is thought to be necessary to devise even more effective countermeasures in order to stabilize the electrolytic phosphate chemical treatment bath and prevent the formation of a waste product in the form of sludge.
As has been described above, the prior art relating to electrolytic phosphate chemical treatment was inadequate with respect to not allowing the reaction of solution phase components (not allowing the formation of sludge), which is the basis of electrolytic surface treatment technology. For this reason, the electrolytic phosphate chemical treatment technology of the prior art was inadequate as an electrolytic surface treatment technology.